| A photometric study of archived images obtained by the Hubble Space Telescope (HST) has been undertaken in support of the NASA-sponsored Kepler search for extrasolar planets. This Discovery mission will monitor 100,000 nearby Main Sequence stars along the Cygnus arm of the Galaxy with apparent visual magnitudes between 9 and I4 for transits by Earth-size planets with relatively short orbital radii. To achieve the necessary photometric precision and minimize noise due to spacecraft motion, Kepler will intentionally defocus target PSFs to 0.18 arcmin2. However, a consequence of this design feature is the inclusion of background luminous sources in the measured flux of a target star. If such a background source were an eclipsing binary system with apparent visual magnitude between 18 and 25, the resulting periodic variation in flux could be misinterpreted as a planetary transit for low signal-to-noise events. To determine the statistical significance of this issue, HST WFPC2 data obtained from the STScI archive have been reduced. From these data, the variation in total star counts as a function of Galactic latitude was measured. The maximum surface density of faint stars per arcmin2 was found to be approximately 260 ± 77. Under assumptions made on the geometric probability of an observed star being an eclipsing binary, the maximum surface density of stars which could be misinterpreted as planetary transits by Kepler was estimated to be approximately 11 ± 3. The presence of eclipsing binary optical doubles in the Kepler field of view is therefore a potentially significant source of false positive signals. The issue can be mitigated by including all photometric information in the data stream sent by the spacecraft, and by studying the areas around Kepler target stars with ground-based telescopes and the HST. |
